This proposal deals with how serotonin modulates the responses of single neurons in a midbrain auditory nucleus to both simple and complex auditory stimuli. The health relatedness of this project arises from the fact that the auditory system of the experimental animal is typically mammalian in most regards, and that some of the auditory stimuli to be used frequency-modulated sweeps, are important components of human speech. In this way, the proposed experiments can be extrapolated to the human auditory system at some level, and will provide basic knowledge in an area about which little is known: serotonergic modulation of auditory processing. I will investigate three main issues. l). How serotonin modulates the excitatory and inhibitory responses of cells to pure tones of varying frequency and intensity. 2). How serotonin correspondingly modulates responses to more spectrally and temporally complex sweeps through multiple frequencies. 3). Which serotonin receptors mediate cell responses to pure tones and to frequency- modulated sweeps. What will emerge is a picture of how serotonin modulates the responses of single neurons to complex, multi-frequency sounds through modulating the simpler elements of the excitatory and inhibitory responses to single frequencies.